The present invention relates to a lithium secondary battery comprising a positive electrode having a lithium-containing composite oxide as active material, a negative electrode having graphite capable of reacting to insert or extract lithium as active material, a separator, and an organic electrolyte solution.
A lithium ion secondary battery comprising an organic electrolyte solution, a negative electrode active material of carbon material, and a positive electrode active material of lithium-containing composite oxide has a higher voltage, a higher energy density, and an excellent low temperature characteristic as compared with a secondary battery of aqueous solution. In such secondary battery, since lithium metal is not used as negative electrode, an excellent cycle stability and a high safety are known, and it is put in practical use. Further, a lithium secondary battery mixing a polymer absorbing and holding an organic electrolyte solution in the active material layer and using a separator made of such polymer is being developed as thin and lightweight type.
Various additives have been proposed so far for improving the characteristics of the lithium secondary battery. In particular, it is known that 12-crown-4-ether as crown ether (coronand) and lithium ion strongly form a complex of 1:1, and it has been proposed to use the crown ether as an additive for suppressing lithium dendrite (for example, Japanese Patent Publication No. 58-12992, Japanese Laid-open Patent No. 57-141878, Japanese Laid-open Patent No. 61-8849, Japanese Patent No. 2771406, U.S. Pat. Nos. 4,132,837, and 4,520,083). As a stabilizing agent in LiAs/F6/THF system, 18-crown-6-ether has been proposed (Proc. 34th Int. Power Sources Symp., 84, IEEE, Piscataway, N.J.).
Also in the Li/TiS2 battery system utilizing intercalation reaction, the additive effect of 12-crown-4-ether has been reported (J. Electrochem. Soc., 134-(1987), 2107). Further, Japanese Laid-open Patent No. 6-13110 proposes to use crown ether as cosolvent or additive in the battery system utilizing the intercalation reaction to the negative electrode using graphite, and 12-crown-4-ether is disclosed as an optimum crown ether for lithium cation. The content of such 12-crown-4-ether in molar number is required to be a molar number equal to superior to that of electrolyte salt, or preferably a double molar number of electrolyte salt.
Besides, as an additive for promoting intercalation reaction into graphite in the PC system electrolyte solution, 12-crown-4-ether has been proposed (J. Electrochem. Soc., 140 (1993), 922; J. Electrochem. Soc., 140 (1993), L101; J. Electrochem. Soc., 141 (1994), 603). Further, in the polymer battery system, similar proposals have been made (Japanese Laid-open Patent No. 61-284071, Japanese Laid-open Patent No. 3-220237, U.S. Pat. Nos. 4,609,600, and 5,523,179, etc.).
As clear from these proposals, the state of solvation varies significantly depending on the complex forming capacity and complex forming of crown ether in the organic electrolyte solution system on lithium ions. However, depending on the type of the crown ether, the complex may be too strong, or the effect of complex formation varies depending on the type of the electrolyte solution solvent, and it is not put in practical use yet.
A lithium ion secondary battery of organic electrolyte solution system comprising a positive electrode active material having a lithium-containing composite oxide, a negative electrode active material having a graphite material capable of reacting to insert or extract lithium has problems such as drop of capacity in high temperature storage. For example, when a conventional lithium ion secondary battery is stored for 10 days at 70xc2x0 C., the battery capacity deteriorates to about 70%.
It is hence an object of the invention to present a lithium secondary battery of high energy density or a lithium polymer secondary battery having such characteristics as small capacity drop in high temperature storage, favorable cycle characteristic and excellent reliability.
A lithium secondary battery of the invention comprises a positive electrode having an oxide containing lithium as a positive electrode active material, a negative electrode having a negative electrode active material capable of reacting chemically with the lithium, an electrolyte solution, a separator, and a ligand.
The electrolyte solution contains a solvent, and an electrolyte dissolved in the solvent, and the solvent has about 20 donor number or less.
The ligand is oriented at the interface of the electrolyte solution and positive electrode surface, and at the interface of the electrolyte solution and negative electrode surface, the ligand has a stronger coordination selecting capacity than the solvent or electrolyte against the lithium, and the ligand has a cyclic structure having a pore in the chemical formula, and this pore has a diameter of about 1.7 angstroms or more.
The ligand is contained in the electrolyte solution, and the ligand is contained in a range of 10xe2x88x921 to 10xe2x88x924 by molar ratio to the electrolyte.
Or, the ligand is contained in the electrolyte solution, and the ligand is contained in a range of 1 micromole to 1 millimole per 1 Ah of the battery capacity.
Preferably, the solvent contains one mixed solvent of (a) cyclic carbonate and chain carbonate, or (b) cyclic carbonate, chain carbonate, and aliphatic carboxylic acid ester, and the amount of the mixed solvent contains more than 80% of the total volume of the solvent.
Preferably, the electrolyte contains at least one selected from the group consisting of lithium perchlorate, lithium tetrafluoro borate, lithium hexafluoro phosphate, trifluoromethane sulfonate, and trifluoromethane imide sulfonate.
Preferably, the ligand is at least one selected from the group consisting of coronand, podanocoronand, cryptand, and spherand.
Preferably, the oxide containing lithium has lithium cobalt oxide, and the negative electrode active material has graphite.
In this composition, deterioration of battery characteristic after the secondary battery is stored in high temperature atmosphere is remarkably improved, and even after storage in high temperature atmosphere, a high reliability and a high energy density are maintained.